1. Field of the Invention
The present invention concerns generation of a color look-up table (LUT) by which colors in a first color space (such as RBG or CMYK or other device dependent color space) are converted to or from a second color space (such as CIELAB or other device independent color space). Specifically, the invention concerns generation of such a color look-up table at regularized grid intervals, without requiring an actual empirical color measurement at each grid point.
2. Description of the Related Art
Conventional color systems often employ look-up tables (LUTs) for conversion between color spaces. For example, it is conventional to employ a LUT so as to convert device independent colors to printer output colorant values for printout by a color printer.
Such LUTs are conventionally built by color measurement of sample outputs of the device for a reasonably sized sample of color patches in the device color space. Preferably, such color patches are uniformly spaced at regular intervals along the device color space. The reason that the samples are uniformly spaced at regular intervals across the device color space is that when accessing entries in the LUT, computer throughput and speed is increased greatly since color coordinates for desired colors can easily be mapped into the LUT. In contrast, non-uniformly spaced LUTs require a pre-processing step in order to map from a desired color to a grid point in the LUT, and it is this pre-processing step that greatly reduces speed and throughput when accessing the LUT.
On the other hand, certain regions of a device's color space are critical in the sense that good color reproduction is required (such as for flesh tones), or in the sense that large non-linearities occur even with small changes in device color coordinates. For such regions, it is desired to obtain more color patches by decreasing the interval between samples, so as to obtain greater color accuracy and fidelity in these critical regions. However, if a regularized LUT is desired, then in order to increase the sampling just in a few critical regions, it is also necessary to increase sampling throughout the entire colorant space. Such an increase is ordinarily time-consuming and inefficient, because of the need to make measurements at each new grid point. For example, to increase sampling from an 8.times.8.times.8.times.8=4,096 patch LUT, to a 9.times.9.times.9.times.9=6,561 color patches, an additional 2,465 color patches are required, with corresponding color measurements.
U.S. Pat. No. 5,594,557 to Rolleston proposes one approach for increasing color accuracy in certain critical regions. According to Rolleston's patent, in regions of local non-linearities, additional data points are provided to a look-up table based on empirically measured colors together with calculated colors that have not been measured. Two drawbacks remain in Rolleston's method. First, the resulting printer LUT is not regular in the sense that the interval between grid points is not fixed. Thus, access to entries in the LUT requires a pre-processing map between a color value and a corresponding address in the LUT, thereby slowing access. Second, the resulting LUT is not completely filled. Thus, a more complicated interpolation involving non-uniform interpolation is required in post-processing for colors not found identically on the LUT grid.